Light, compact, electric and electronic appliances including cellular phones, laptop computers, and camcorders have recently been vigorously developed and produced. These portable electric and electronic appliances operate on battery packs when separate power supplies are unavailable. Battery packs comprise at least one battery to generate the voltage necessary to operate the appliances for a specified period of time.
Secondary batteries have recently been used for these battery packs because they can be charged and recharged, making them economical. Typical examples of secondary batteries include nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries, and lithium ion batteries. Lithium secondary batteries are particularly attractive because they operate at 3.6 V, a voltage three times higher than that of nickel-cadmium batteries and nickel-hydrogen batteries which are widely used as power supplies for portable electronic appliances. In addition, lithium secondary batteries have high energy density per unit weight.
Lithium secondary batteries use lithium-based oxides as positive electrode active materials and carbon materials as negative electrode active materials. Lithium secondary batteries are generally classified according to the type of electrolyte used and are classified into lithium ion batteries using liquid electrolytes and lithium polymer batteries using polymer electrolytes. Lithium secondary batteries can take various shapes, including cylinders, squares, and pouches.
A lithium secondary battery generally comprises a positive electrode plate coated with a positive electrode active material, a negative electrode plate coated with a negative electrode active material, and a separator positioned between the positive and negative electrode plates. The separator serves to prevent short circuits and allows passage only to lithium ions. An electrode assembly is formed by winding the positive electrode plate, the negative electrode plate and the separator. The secondary battery also comprises a battery case for holding the wound electrode assembly. An electrolyte is injected into the lithium secondary battery case to allow movement of lithium ions.
The electrode assembly of the lithium secondary battery is formed by first applying positive and negative electrode active materials to positive and negative electrode collectors, respectively, which have metallic bases. The positive and negative electrode plates are then wound together with a separator to form the electrode assembly.
However, these wound electrode assemblies may become unwound due the elasticity of the metal electrode collectors. The elastic force of the metal causes the metal electrode collectors, and thus the electrode assemblies, to unwind. To prevent the assembly from unwinding in this fashion, winding fixation tapes wound around the outer periphery of the electrode assembly have been proposed. These winding fixation tapes are wound around the electrode assembly in the same direction as the electrode assembly itself is wound.
However, these winding fixation tapes cannot prevent swelling of the electrode assembly, which occurs upon erroneous battery operation, e.g., overcharging. Such erroneous operation causes the electrode assembly to swell during charging and generates a short circuit between the positive and negative electrode plates. Consequently, the thermal stability of the electrode assembly during charging deteriorates.